A vapor compression system consists of a compressor, a heat rejection heat exchanger or gas cooler, an expansion device, and an evaporator. Economizer cycles are sometimes employed to increase the efficiency and/or capacity of the system. Economizer cycles operate by expanding the refrigerant leaving the heat rejecting heat exchanger to an intermediate pressure and separating the refrigerant flow into two streams. One stream is sent to the heat absorbing heat exchanger, and the other is sent to cool the flow between two compression stages. In one form of an economizer cycle, a flash tank is used to perform the separation. In an economizer cycle with flash tank, a refrigerant discharged from the gas cooler passes through a first expansion device, and its pressure is reduced. Refrigerant collects in the flash tank as part liquid and part vapor. The vapor refrigerant is used to cool refrigerant exhaust as it exits a first compression device, and the liquid refrigerant is further expanded by a second expansion device before entering the evaporator. Such a flash tank economizer is particularly useful when operating in transcritical conditions, such as are required when carbon dioxide is used as the working fluid, and is described in U.S. Pat. No. 6,385,980, assigned to the assignee of the present invention. In the non-economized mode the vapor line connecting the flash tank with the compressor mid-stage is closed and the entire refrigerant mass flow rate entering the flash tank is directed to the second expansion stage.
When the system operates in the economized mode, it is desirable to prevent the reversal of the flow direction in the economizer vapor line, e.g., from the compressor to the flash tank. That is, if the pressure in the compressor mid-stage is higher than in the flash tank, the flow direction in the economizer vapor line will be reversed, resulting in flow from the compressor through the economizer vapor line into the flash tank. Flow reversal in the economizer vapor line reduces the system cooling capacity and energy efficiency. Flow reversal will generally result when the compressor mid-stage pressure exceeds the pressure in the flash tank and can occur at certain operating conditions, dictated by the temperature at the heat sink and heat source and the specifics of the system design, such as heat exchanger size and compressor size.
In U.S. Pat. No. 6,202,438, assigned to Scroll Technologies, a former subsidiary of the present assignee, there is disclosed an economized refrigeration circuit with a check valve disposed within the compressor to prevent the return flow of refrigerant from the compressor to the economizer. However, that check valve is employed only for that purpose, and a separate economizer valve is employed to turn the economizer on or off. Further, the economizer is not of the flash tank type, and the manner in which it operates is different from the flash tank economizer of the present invention.
Due to the thermophysical properties of CO2, the refrigeration system can operate in both the subcritical and transcritical modes. The subcritical mode is similar to the operation of systems with conventional refrigerants. In the transcritical mode the refrigerant pressure in the heat rejection heat exchanger, and possibly in the flash tank, is above the critical pressure, while the evaporator operates as in the subcritical mode. If the flash tank pressure is above the critical pressure, the separation of the refrigerant into liquid and vapor phases will not occur as desired since a supercritical fluid does not form a distinct liquid and vapor phase.